EcoSAR is an advanced airborne polarimetric and interferometric P-band SAR (Synthetic Aperture Radar) instrument in development at NASA/GSFC through NASA's IIP (Instrument Incubator Program). The objective is to provide unprecedented two- and three dimensional fine scale measurements of terrestrial ecosystem structure and biomass. These measurements directly support the science requirements for the study of the carbon cycle and its relationship to climate change, recommended by the National Science Foundation's Decadal Survey (2007) and highlighted in NASA's Plan for a Climate-Centric Architecture (2010). 1)2)3)

Science requirements: EcoSAR will be capable of providing estimates of canopy height at 1 m resolution and estimates of above-ground woody biomass to an accuracy of 20 Mg/ha (Megagram/hectare) or 20%. The instrument will also be able to map areas of forest extent and areas of disturbance. The finest resolution of the EcoSAR data will be at 0.5 m in the azimuth and 0.75 in the slant range. In addition to quantifying biomass, ecosystem structure, extent disturbance and recovery, the EcoSAR instrument will also be able to perform mapping of ice sheets, ice dynamics and thickness, permafrost depth, flux and storage of water, which is directly in line with Decadal Survey objectives. 4)5)

EcoSAR will employ electronic beamsteering on transmit and digital beamforming on receive similar to that proven by DBSAR (Digital Beamforming SAR), permitting the implementation of advanced imaging techniques which overcome problems inherent in conventional SAR systems.

The EcoSAR architecture will allow considerable measurement flexibility such as post processing synthesis of multiple beams, simultaneous measurement over both sides of the flight track, and variable incidence angles (Figure 2).

Beams can be processed with different beamwidths and side lobe levels to control swath widths and minimize side lobe contamination. Ambiguities in the receiver signal can be suppressed by post processing by appropriate nullsteering of the antenna pattern.

EcoSAR operates at a center frequency of 435 MHz (69 cm wavelength) and features a fully programmable bandwidth over a 200 MHz range. Operational modes with slant range resolutions from 5 m (30 MHz bandwidth) to 25 m (6 MHz bandwidth) are being used as a nominal mode in frequency restricted areas, and a science mode with slant range resolutions as fine as 0.75 m (up to 200 MHz bandwidth) is employed in authorized or remote areas.

EcoSAR will perform cross-track interferometry in either "standard" mode or in "ping pong" mode employing two array antennas. The antennas will have dimensions of 3 m x 1 m and will be mounted under the aircraft wings (Figure 3). This configuration will provide a 25 m baseline in standard mode and a 50 m baseline in "ping-pong" mode. These baselines are a small fraction of the critical baseline, therefore, minimizing any decorrelation due to the baseline.

Note: In ping-pong mode, the radar transmits alternatively out of the two antennas and receives the radar echo only through the same antenna. RTIs (Repeat-Track Interferometers) are inherently operating in "ping-pong" mode. 6)

• The element design adopted is based on stacked patch approach using a process of modeling, simulations, prototyping, and testing which led to the final element.

Parameter

Value

Parameter

Value

Antenna type

Stacked patch

Scan angle

± 35o

Center frequency

435 MHz

Cross polarization isolation

>30 dB

Maximum bandwidth

200 MHz

Number of subarrays

10

Gain

19 dB

Total number of elements

20

Polarization

Dual, linear

Dimensions

3 m x 1 m

Table 2: Antenna parameters

The EcoSAR antenna elements will be based on a new microstrip design approach which increases bandwidth and cross-polarization isolation. Antenna element simulations have demonstrated to support 50% bandwidth and polarization isolation better than 25 dB. The EcoSAR antennas are being designed and fabricated by the Aerospace Department at the University of Kansas. The two antenna arrays have dimensions of 3 m x 1 m x 19 cm.

The antenna arrays will be connected to a REU (Radar Electronics Unit) which resides in the aircraft fuselage; it includes 32 dual-polarity T/R (Transmit/ Receive) modules. The T/R modules will equipped with 20 W SSPAs (solid state power amplifiers), LNAs (Low Noise Amplifiers), circulators, couplers, filters, and control switches, as illustrated in Figure 5. They will also include closed loops for robust calibration, dynamic beam control and adaptive waveform generation.

The analysis on the types of architectures that best suit the EcoSAR system led the project to select a hybrid design where the transmitter portion of the transceiver will be realized using connectorized components, while the receiver will be designed on a PCB (Printed Circuit Board) with surface mount components. This approach yields the optimum performance in terms of power, size, and efficiency.

- The power switching capability of the SSPA enable a relatively simple cooling system based on heat sinks and air cooling.

- Considerations for the mechanical design included physical mounting and vibration isolation, physical isolation of each channel for EMC/EMI purposes, heat dissipation from the power amplifier, physical access to facilitate interconnection, testing and maintenance.

The EcoSAR architecture will support full polarimetric operation and a hybrid polarity operation (i.e., transmit circular, receive horizontal and vertical polarizations) for the retrieval of the full backscattering matrix. 7) Orthogonal polarity waveform generation techniques will also be used to implement RFI (Radio Frequency Interference) mitigation techniques.

EcoSAR's digital beamforming, polarimetric, and "single pass" interferometric operation will make this system a first of its kind, providing unprecedented two- and three-dimensional fine scale measurements of terrestrial ecosystem structure and biomass. Terrestrial ecosystems are a crucial component of the carbon cycle, and the greatest uncertainty in the global carbon cycle stems from the estimation of carbon uptake and release by terrestrial ecosystems. EcoSAR will map forest cover, above ground biomass, disturbance due to deforestation and logging, forest recovery, and wetland inundation, closing the gap in understanding the global carbon cycle.

It is expected that the TRL (Technology Readiness Level) of the whole system, as well as its main elements, be at 6 at the completion of the work when the EcoSAR performs flight campaigns.

The information compiled and edited in this article was provided byHerbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (herb.kramer@gmx.net).